[SDK] Allow metric instrument names to contain / characters

[marcalff]

Fixes #2303

Changes

Allow / in metric instrument names.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2310 (70f8524) into main (f5f3934) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2310      +/-   ##
==========================================
- Coverage   87.53%   87.52%   -0.01%     
==========================================
  Files         199      199              
  Lines        5981     5981              
==========================================
- Hits         5235     5234       -1     
- Misses        746      747       +1     

Files Changed	Coverage
sdk/src/metrics/instrument_metadata_validator.cc	ø